cmd/compile: remove implicit deref from len(p) where p is ptr-to-array

func f() *[4]int { return nil }
_ = len(f())

should not panic. We evaluate f, but there isn't a dereference
according to the spec (just "arg is evaluated").

Update #72844

Change-Id: Ia32cefc1b7aa091cd1c13016e015842b4d12d5b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/658096
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Keith Randall <khr@google.com>

2 files changed, 73 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/typecheck/expr.go b/src/cmd/compile/internal/typecheck/expr.go
index 2eec8d41ad..44a69f0332 100644
--- a/src/cmd/compile/internal/typecheck/expr.go
+++ b/src/cmd/compile/internal/typecheck/expr.go
@@ -634,16 +634,16 @@ func tcIndex(n *ir.IndexExpr) ir.Node {
 func tcLenCap(n *ir.UnaryExpr) ir.Node {
 	n.X = Expr(n.X)
 	n.X = DefaultLit(n.X, nil)
-	n.X = implicitstar(n.X)
 	l := n.X
 	t := l.Type()
 	if t == nil {
 		n.SetType(nil)
 		return n
 	}
-
 	var ok bool
-	if n.Op() == ir.OLEN {
+	if t.IsPtr() && t.Elem().IsArray() {
+		ok = true
+	} else if n.Op() == ir.OLEN {
 		ok = okforlen[t.Kind()]
 	} else {
 		ok = okforcap[t.Kind()]
diff --git a/test/fixedbugs/issue72844.go b/test/fixedbugs/issue72844.go
new file mode 100644
index 0000000000..0322841ded
--- /dev/null
+++ b/test/fixedbugs/issue72844.go
@@ -0,0 +1,70 @@
+// run
+
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+//go:noinline
+func nilPtrFunc() *[4]int {
+	return nil
+}
+
+var nilPtrVar *[4]int
+
+func testLen1() {
+	_ = len(*nilPtrFunc())
+}
+
+func testLen2() {
+	_ = len(nilPtrFunc())
+}
+
+func testLen3() {
+	_ = len(*nilPtrVar)
+}
+
+func testLen4() {
+	_ = len(nilPtrVar)
+}
+
+func testRange1() {
+	for range *nilPtrFunc() {
+	}
+}
+func testRange2() {
+	for range nilPtrFunc() {
+	}
+}
+func testRange3() {
+	for range *nilPtrVar {
+	}
+}
+func testRange4() {
+	for range nilPtrVar {
+	}
+}
+
+func main() {
+	//shouldPanic(testLen1)
+	shouldNotPanic(testLen2)
+	shouldNotPanic(testLen3)
+	shouldNotPanic(testLen4)
+	//shouldPanic(testRange1)
+	shouldNotPanic(testRange2)
+	shouldNotPanic(testRange3)
+	shouldNotPanic(testRange4)
+}
+
+func shouldPanic(f func()) {
+	defer func() {
+		if e := recover(); e == nil {
+			panic("should have panicked")
+		}
+	}()
+	f()
+}
+func shouldNotPanic(f func()) {
+	f()
+}